A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
It has been proposed to immerse the substrate in the lithographic projection apparatus in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final optical element of the projection system and the substrate. In an embodiment, the liquid is distilled water, although another liquid can be used. The point of this is to enable imaging of smaller features since the exposure radiation will have a shorter wavelength in the liquid. The effect of the liquid may also be regarded as increasing the effective numerical aperture (NA) of the system and also increasing the depth of focus.
The substrate is conventionally clamped to a substrate holder during exposures. Two clamping techniques are commonly used. In vacuum clamping a pressure differential across the substrate is established, e.g. by connecting the space between the substrate to an under-pressure, so that a higher pressure above the substrate exerts a force holding the substrate to the substrate holder. In electrostatic clamping, electrostatic forces are used to exert a force between the substrate and the substrate holder. Several different arrangements are known to achieve this. In one arrangement a first electrode is provided on the lower surface of the substrate and a second electrode on the upper surface of the substrate holder. A potential difference is established between the first and second electrodes. In another arrangement two semi-circular electrodes are provided on the substrate holder and a conductive layer is provided on the substrate. A potential (voltage) difference is applied between the two semi-circular electrodes so that the two semi-circular electrodes and the conductive layer on the substrate act like two capacitors in series.
A substrate holder conventionally has a plurality of burls to support the substrate. The total area of the burls that contacts the substrate is small compared to the total area of a substrate. Therefore, the chance that a contaminant particle randomly located on the surface of the substrate or the substrate holder is trapped between a burl and the substrate is small. Also, in manufacture of the substrate holder, it is easier to make the tops of the burls accurately coplanar, than to make a large surface accurately flat.
To load a substrate onto a substrate holder in preparation for exposure it is placed by a substrate handler robot onto so-called e-pins which project through the substrate holder. The e-pins then retract to lower the substrate onto the substrate holder. Then, the clamping force is applied so that the substrate is held very firmly during exposure. The clamping force is large enough to hold the substrate in position even when it is subjected to very large accelerations and to resist thermal expansion, for example due to absorption of energy from the projection beam during exposure. If the substrate is distorted, e.g. so as to be convex, the clamping force will tend to flatten out the substrate against the substrate holder.